Biofuels have a long history ranging back to the beginning of the 20th century. As early as 1900, Rudolf Diesel demonstrated at the World Exhibition in Paris, France, an engine running on peanut oil. Soon thereafter, Henry Ford demonstrated his Model T running on ethanol derived from corn. Petroleum-derived fuels displaced biofuels in the 1930s and 1940s due to increased supply, and efficiency at a lower cost.
Market fluctuations in the 1970s, due the Arab oil embargo and the Iranian revolution, coupled to the decrease in US oil production, led to an increase in crude oil prices and a renewed interest in biofuels. Today, many interest groups, including policy makers, industry planners, aware citizens, and the financial community, are interested in substituting petroleum-derived fuels with biomass-derived biofuels. The leading motivation for developing biofuels is of economical nature, namely, the threat of ‘peak oil’, the point at which the consumption rate of crude oil exceeds the supply rate, thus leading to significantly increased fuel cost results in an increased demand for alternative fuels.
Biofuels tend to be produced with local agricultural resources in many, relatively small facilities, and are seen as a stable and secure supply of fuels independent of geopolitical problems associated with petroleum. At the same time, biofuels enhance the agricultural sector of national economies. In addition, since fossil sources of fuels take hundreds of millions of years to be regenerated and their use increases carbon dioxide levels in the atmosphere, leading to climate change concerns, sustainability is an important social and ethical driving force which is starting to result in government regulations and policies such as caps on carbon dioxide emissions from automobiles, taxes on carbon dioxide emissions, and tax incentives for the use of biofuels.
The acceptance of biofuels depends primarily on economical competitiveness of biofuels when compared to petroleum-derived fuels. Biofuels that cannot compete in cost with petroleum-derived fuels will be limited to specialty applications and niche markets. Today, the use of biofuels is limited to ethanol and biodiesel. Currently, ethanol is made by fermentation from corn in the US, sugar cane in Brazil, and other grains worldwide. Ethanol is competitive with petroleum-derived gasoline, exclusive of subsidies or tax benefits, if crude oil stays above $50 per barrel. Biodiesel has a breakeven price of crude oil of over $60/barrel to be competitive with petroleum-based diesel (Nexant Chem Systems, 2006, Final Report, Liquid Biofuels: Substituting for Petroleum, White Plains, N.Y.).
Several factors influence the core operating costs of a carbohydrate based biofuel source. In addition to the cost of the carbon-containing, plant produced raw material, a key factor in product economic costs for ethanol or other potential alcohol based biofuels, such as butanol, is the recovery and purification of biofuels from aqueous streams. Many technical approaches have been developed for the economic removal of alcohols from aqueous based fermentation media. The most widely used recovery techniques today use distillation and molecular sieve drying to produce ethanol. For example, butanol production via the Clostridia-based acetone-butanol-ethanol fermentation also relied on distillation for recovery and purification of the products. Distillation from aqueous solutions is energy intensive. For ethanol, additional processing equipment to break the ethanol/water azeotrope is required. This equipment, molecular sieves, also uses significant quantities of energy.
Many unit operations have been studied for the recovery and purification of fermentation produced alcohols, including filtration, liquid/liquid extraction, membrane separations (e.g., tangential flow filtration, pervaporation, and perstraction), gas stripping, and “salting out” of solution, adsorption, and absorption. Each of the approaches has advantages and disadvantages depending on the circumstances of the product to be recovered and the product's physical and chemical properties and the matrix in which it resides.
Variables which control the production costs of biofuels can be characterized as those impacting operating costs, capital costs, or both. Typically, key variables that control fermentation economic performance include carbohydrate yield to desired product, product concentration and volumetric productivity. All three key variables, yield, product concentration, and volumetric productivity, impact both capital and operating costs.
As product yield on carbohydrate fermented is increased, the production costs for a given unit of product decrease linearly relative to raw material costs. The product yield on carbohydrate also impacts equipment size, capital expenditures, utilities consumption and feed stock preparation materials such as enzymes, minerals, nutrients (vitamins), and water. For example an increase in product yield on glucose to butanol from 50% to 90% of theoretical results in a 44% decrease in direct operating costs. Also, the increased yield of 90% reduces the amount of raw materials handled and processed. The increased yield directly reduces capital investment required for the production facility as all equipment from carbohydrate preparation through purification and recovery are reduced in size. Equipment, piping, and utility requirements can be reduced by 32% if yield is increased from 50% to 90%. The direct influence of product yield on production costs makes it a key influence on the cost and market viability for biofuels. An approach to increase product yield involves Genetically Engineered Microorganisms (GEMs) that can be constructed to manipulate the organism's metabolic pathway to reduce or eliminate undesired products, increase the efficiency of the desired metabolite or both. This allows for the deletion of one or both of low cost products and undesired products, which increases production of desired products.
For example, US Patent Application Publication 2005/0089979 discloses a fermentation process that utilizes a Clostridium beijerinckii microorganism that produces a mixture of products including 5.3 g/L acetone, 11.8 g/L butanol, and 0.5 g/L ethanol. An appropriately modified Genetically Engineered Microorganism eliminates acetone and ethanol production while increasing conversion of carbohydrates to butanol. The redirection of a carbohydrate feedstock away from ethanol and acetone to butanol increases butanol production from 11.8 g/L to 18.9 g/L, a 60% increase in butanol production relative to carbohydrate consumption. The elimination of the ethanol and acetone byproducts also allows for reduced capital costs as less equipment is necessary to complete recovery and purification.
Application of biochemical tools, including, genetic engineering and classical strain development can also impact the final product concentration (g/L) and fermentation volumetric productivity (g/L-hr) of the biocatalyst. Final product concentration and volumetric productivity impacts several aspects of product economics, including equipment size, raw material use, and utility costs. As the tolerable product concentration increases in the fermentation, recovery volumes of aqueous solutions are decreased which results in reduced capital costs and smaller volumes of materials to process within the production facility.
Volumetric productivity directly impacts the required fermentor capacity to achieve the same product output. For example, a traditional Clostridium beijerinckii acetone-butanol-ethanol (ABE) fermentation produces a ratio of acetone, butanol, and ethanol. Genetically engineered microbes allow the designed production of a single product, such as n-butanol, isobutanol or 2-butanol (Donaldson et al., U.S. patent application Ser. No. 11/586,315). Butanol tolerant hosts can be identified utilizing techniques to identify and enhance the butanol tolerance (Bramucci et al., U.S. patent application Ser. No. 11/743,220). These two techniques can then be combined to produce butanol at commercially relative concentrations, and volumetric productivity.
The utilization of GEMs to increase product volumetric productivity and concentration may strongly influence product economics. For example, a butanol fermentation completed at twice the volumetric productivity will reduce fermentor cost by almost 50% for a large industrial biofuels fermentation facility. The fermentor capital cost and size reduction decreases depreciation and operating costs for the facility. Similarly, if the GEMs result in an organism that is tolerant to higher butanol concentrations, operating and capital costs are reduced for a given production volume. For example, if a wild type strain is capable of tolerating 20 g/L butanol and a corresponding genetically improved or genetically enhanced microorganism tolerates 40 g/L butanol, the water load in the fermentor broth volume handled in downstream recovery and purification equipment is reduced by half. In this example, the doubling of product concentration in the fermentation broth almost halves the amount of water to be recovered and processed in recovery unit operations.
A large number of minor cost components also impact operating and capital costs for biofuels production. Example factors that can impact fermentation include, but are not limited to, chemical additives, pH control, surfactants, and contamination are some of the factors but many additional factors can impact fermentation product cost.